(a) Field of the Invention
The present invention relates to a ligand compound, an organic chromium compound, a catalyst system for ethylene oligomerization, a preparation method thereof, and an ethylene oligomerization method using the same. More particularly, the present invention relates to a ligand compound, an organic chromium compound, a catalyst system for ethylene oligomerization, a preparation method thereof, and an ethylene oligomerization method using the same, in which they can be used to prepare a low-density polyethylene in one reactor by using a small amount of comonomers or by using only ethylene without comonomers, and in particular, they maintain high catalytic activity and high alpha-olefin selectivity even though supported on a support.
(b) Description of the Related Art
Linear alpha-olefin is widely used in important commercial substances such as comonomers, detergents, lubricants, plasticizers or the like, and in particular, 1-hexene and 1-octene are used for polymerization of olefins having various physical properties by one catalyst.
They are commonly used as comonomers for controlling density of polyethylene during preparation of linear low density polyethylene (LLDPE).
In the conventional preparation process of LLDPE (Linear Low-Density Polyethylene), copolymerization of ethylene with alpha-olefin, for example, a comonomer such as 1-hexene and 1-octene is carried out in order to control density by forming branches in the polymer backbone.
Therefore, there is a problem that the comonomer increases the production cost of LLDPE having a high content of comonomers. Many different methods have been tried to solve this problem.
Further, because the application field or market size depends on the type of alpha-olefin, a technique capable of selectively producing a particular olefin is commercially important. Recently, many studies have been conducted on a chromium catalyst for preparing 1-hexene or 1-octene with a high selectivity through selective ethylene oligomerization.
The conventional commercial method for 1-hexene or 1-octene preparation is the SHOP process of Shell Chemical, the Ziegler process of Chevron Philips, or the like, which is used to produce alpha-olefins having a wide distribution ranging from 4 to 20 carbons.
A chromium-based catalyst for ethylene trimerization having a ligand of the formula (R1)(R2)X—Y—X(R3)(R4) has been suggested, in which X is phosphorus, arsenic or antimony, Y is a linking group such as —N(R5)—, and at least one of R1, R2, R3 and R4 has a polar substituent or an electron donating substituent.
Further, studies have been conducted on (o-ethylphenyl)2PN(Me)P(o-ethylphenyl)2 as a ligand which shows no catalytic activity for 1-hexene under catalytic conditions and has no polar substituent in at least one of R1, R2, R3 and R4 (Chem. Commun., 2002, 858).
However, the prior ligands containing heteroatoms as described above are still required to consistently maintain their polymerization activity during reactions for producing 1-octene or 1-hexene and to have high selectivity.
Further, there is a problem that their activity is greatly reduced when they are supported on a support such as silica (J. Am. Chem. Soc 2003, 125, 5272, Ind. Eng. Chem. Res. 2008, 47, 5369).